1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a plurality of fuel cell units that are formed by sandwiching an electrode assembly between a pair of separators.
2. Description of the Related Art
Among fuel cell units, there is one type that is formed in a plate shape by sandwiching between a pair of separators an electrode assembly that is formed by placing an anode electrode and a cathode electrode respectively on either side of a solid polymer electrolyte membrane. A fuel cell is formed by stacking in the thickness direction of the fuel cell units a plurality of fuel cell units that are structured in this way.
In each fuel cell unit there is provided a flow passage for fuel gas (for example, hydrogen) on the surface of the anode side separator that is positioned facing the anode electrode, and there is provided a flow passage for oxidizing gas (for example, air that contains oxygen) on the surface of the cathode side separator that is positioned facing the cathode electrode. In addition, a flow passage for a cooling medium (for example, pure water) is provided between adjacent separators of adjacent fuel cell units.
When fuel gas is supplied to the electrode reaction surface of the anode electrode, hydrogen is ionized here and moves to the cathode electrode via the solid polymer electrolyte membrane. Electrons generated during this reaction are extracted to an external circuit and used as direct current electrical energy. Because oxidizing gas is supplied to the cathode electrode, hydrogen ions, electrons, and oxygen react to generate water. Because heat is generated when water is created at the electrode reaction surface, the electrode reaction surface is cooled by a cooling medium made to flow between the separators.
The fuel gas, oxidizing gas (generically known as reaction gas), and the cooling medium each need to flow through a separate flow passage. Therefore, sealing technology that keeps each flow passage sealed in a fluid-tight or airtight condition is essential.
Examples of portions that must be sealed are: the peripheries of penetrating supply ports formed in order to supply and distribute reaction gas and cooling medium to each fuel cell unit of the fuel cell; the peripheries of discharge ports that collect and discharge the reaction gas and cooling medium that are discharged from each fuel cell unit; the outer peripheries of the electrode assemblies; and the outer peripheries between the separators of adjacent fuel cell units. Organic rubber that is soft yet also has the appropriate resiliency or the like is employed as the material for the sealing member.
In recent years, however, size and weight reduction, as well as a reduction in the cost of fuel cells, have become the main barriers in progress towards the more widespread application of fuel cells through their being mounted in practical vehicles.
Methods that have been considered for reducing the size of a fuel cell include making each fuel cell unit forming the fuel cell thinner, more specifically, reducing the size of the space between separators while maintaining a maximum size for the reaction gas flow passage formed inside each fuel cell unit; and also making the separators thinner.
However, a limit is imposed on how thin the separators can be made by the strength requirements for each separator and by the rigidity requirements for the fuel cell. Reducing the height of the sealing members is effective in reducing the size of the spacing between separators, however, the height of the sealing members needs to be sufficient for the sealing members to be able to be pressed down sufficiently to ensure that the required sealing performance is obtained. Therefore, there is also a limit to how much the height of the sealing members can be reduced.
Furthermore, in a fuel cell unit, although the volume occupied by the sealing members is indispensable in order for the reaction gas and cooling medium to be sealed in, because this space contributes substantially nothing to power generation, it must be made as small as possible.
FIG. 35 is a plan view showing a conventional fuel cell. In FIG. 35 the symbol 107 indicates a communication port such as a fuel gas supply port and discharge port, an oxidizing gas supply port and discharge port, and a cooling medium supply port and discharge port that each penetrate a fuel cell stack 106 in the direction in which separators 109 and 110 are stacked. The symbol 112 indicates an area formed by a plurality of fuel gas flow passages, oxidizing gas flow passages, and cooling medium flow passages running along the separators 109 and 110.
FIG. 36 is a longitudinal cross-sectional view of a conventional fuel cell stack 106 taken along the line F—F in FIG. 35. As can be seen in plan view, in order to make the volume occupied by the sealing member, which does not contribute to power generation, as small as possible, conventionally, by locating gas sealing members 102 and 103, which respectively seal a fuel gas flow passage 100 and an oxidizing gas flow passage 101, together with a cooling surface sealing member 104, which seals a cooling medium flow passage, aligned in a row in the stacking direction of the fuel cell units 105, the outer dimensions in the stacking direction of the fuel cell stack 106 are minimized.
However, the drawback with the fuel cell stack 106 that is structured in this manner is that if the gas sealing members 102 and 103 that seal the flow passages 100 and 101 as well as the cooling surface sealing member 104 are all placed in a row in the stacking direction of the fuel cell unit 105, then the thickness of the fuel cell stack 106 cannot be made less than a value obtained by adding the height of the cooling surface sealing member 104 to the minimum thickness of each fuel cell unit 105, and multiplying this result by the number of fuel cell units stacked in the fuel cell.
In order to explain this more specifically, the discussion will return to FIG. 36. According to FIG. 36, the fuel gas supply port 107 and the fuel gas flow passage 100 that are isolated in a sealed state by the gas sealing members 102 and 103 and by the cooling surface sealing member 104 are connected by a communication path 108. The communication path 108 is provided in the separator 109 so as to detour around, in the thickness direction of the separator 109, the gas sealing member 102 that seals the entire periphery of the fuel gas flow passage 100. Moreover, the separator 110 also has a similar communication path (not shown) near the oxidizing gas communication port (not shown).
Accordingly, each of the separators 109 and 110 are formed relatively thickly in order to form the communication path 108; however, as is seen in the cross section in FIG. 36, at the position of the seal line where each of the sealing members 102 to 104 are placed, the separators 109 and 110 are formed with the minimum thickness needed to ensure the required strength, and it is not possible to make them any thinner.
Moreover, because each of the sealing members 102 to 104 is formed with the minimum height needed to secure the sealing performance, it is not possible to reduce the height of the sealing members 102 to 104 any further.
As a result, although the thickness of the fuel cell stack 106 is found by multiplying the number of stacks by the sum of the minimum thickness of the two separators 109 and 110, the thickness needed to form the communication path 108, the height of the two gas sealing members 102 and 103, the thickness of the solid polymer electrolyte membrane 111, and the height of the cooling surface sealing member 104, because these are all indispensable, it is extremely difficult to achieve any further reduction in thickness.
The present invention was conceived in view of the above circumstances, and it is an object thereof to provide a fuel cell that has been made lighter and smaller by reducing the thickness thereof in the stacking direction, while reliably sealing the respective flow paths using the respective sealing members between the separators and the electrode assemblies that form the fuel cell.